Following a mild head injury (MHI), a person may report a variety of symptoms such as headache, memory disturbance, dizziness, and concentration difficulties. For most persons the symptoms are transient, but some suffer persistent symptoms that can have a major impact on everyday life. It remains poorly understood why some but not others have full recovery after MHI. The aim of this thesis was to investigate outcomes after MHI, with particular focus on neuropsychological functioning, fatigue, and risk of dementia. A related objective was to examine the potential association of a genetic factor, Apolipoprotein (APOE), with MHI outcome. The APOE ?4 allele has been associated with unfavorable outcomes after moderate or severe head injury, but little is known about its influence on outcome after MHI. In Study I and II, data from a population-based longitudinal study were used to compare neuropsychological functioning and fatigue before and after MHI. The results from Study I showed a post-injury decline in neuropsychological performance for ?4-carriers, whereas the performance remained unchanged for non-carriers. Study II showed an increase in self-reported fatigue after MHI for both ?4-carriers and non-carriers, with a more pronounced increase for ?4-carriers. In Study III, a case-control study was conducted to examine whether a history of MHI increased the risk of developing dementia later in life. It was found that MHI alone did not increase the risk, but the combination of MHI and APOE ?4 was associated with increased risk of dementia. Taken together, the studies generally indicate a positive outcome after MHI, but in combination with APOE ?4 even mild head injury may lead to long-lasting negative outcomes. Consideration of pre-injury level of functioning and genetic factors seems critical for a complete understanding of the impact of MHI.